SMPS for B+ discussion

[astouffer]

Heres something else to think about. How big of a choke and capacitor do you need to properly filter out 100hz ripple. Now how big for 1Mhz ripple? Even at 100khz not much noise is going to get through an output transformer with a bandwidth that goes to 50khz if you're lucky.

 

[ackcheng]

I guess with an OTL design, there will be no output transformer, the situation will be a bit different?? Yes? No?

 

[ssanmor]

Ackcheng,
The ss state amplifier we tested on was a LM3886 based amplifier (bridge-parallel mode). Hope this helps.
I know that some of our customers are also testing it with ss amplifiers, mainly their own designs, sorry don't have more details about them.
The SPS80 topology is a half-bridge converter.

Best regards,
Sergio

 

[peufeu]

You could make a tube class D amp...

 

[ssanmor]

Class-D amps require very deep knowledge of the devices you are using to get optimal results. I don't consider myself prepared to develop a Class-D amp based on tubes, if that makes any sense or is possible at all, but thanks for the suggestion.

 

[ackcheng]

I have finally bought 2 SPS80HV for my OTL and did a few test on it.

But first, here are some pic for the 500W linear power supply with 5400uF filtering cap

This picture shows the voltage output of the linear power supply (LPS) when there is no signal input to the OTL amp. B+ voltage measured as 161.5Vwith 0.4V ripple¡Acurrent is 0.3A¡C

 

[ackcheng]

OTL output at 0.35W with 1Khz input signal, B+ with 1khzripple at 1V (this is normal for circlotron circuit, I was told)¡Aat 100Hz, there is 0.4V ripple (this is coming from the AC mains)

 

[ackcheng]

When the amp is fully loaded, voltage on B+ is 147V, current 1.25A, 1Khz ripple at 7.5V, 100Hz ripple at 2V

 

[ackcheng]

Now, we tested the OTL amp with SPS80HV for the B+ of putput tube. one module provide 170V and the other provide -170V

When no input to the OTL amp, B+ at172.5V with virtually ripple! Current = 0.35A

 

[ackcheng]

OTL amp at 0.35W output, with 1Khz signal input. Measured at B+

 

[ackcheng]

Magnified photo: 0.35W output at 1Khz, 1khz ripple at 400mV. A problem with OTL circlotron design

 

[ackcheng]

At full load: 88Woutput, 2Vripple at 100hz¡A5V ripple at 1khz.

 

[ackcheng]

Now, the same amp with SPS80HV and 5400uf filtering cap

0.35W output at 1Kh, measured at B+.

 

[ackcheng]

Magnified view: 0.35W output at 1Khz, measured at B+ : 400mV ripple at 1KHz.

 

[ackcheng]

At full load: 88W output¡A1V ripple at 100Hz, 5V ripple at 1khz

 

[ackcheng]

Attached is the summary of the measurement report

 

[ackcheng]

In summary, SPS80 only drops 7.7V but the linear power supply drops 23V. It is obvious that the SPS80 is far superior in loaded condition. There is some improvement in terms of 100Hz ripple and 1K Hz ripple. With additional 5400uF, there is further improvement by 0.9V.

In terms of sound, the SPS80 is so much better. The bass slam is clear, solid and transparent. It is actually more quiet than the LPS. The background is dark black. There is no need to go back to LPS anymore!

When someone tell me again that SMPS is noisy, now I have proved to myself that a good SMPS is better than LPS in all areas!!

However, I think SMPS is good for OTL as the current output is very large. In our measurement, the current at full load is 1.3A, but in theory, the peak current can reach 4A. However, SMPS may be overkill in transformer output amp!?